1. Field of the Invention
The present invention relates generally to a method and apparatus for measuring formation parameters by transmitting and receiving electromagnetic signals within a logging instrument in an earth borehole. More particularly, the present invention is related to downhole logging tools which use electromagnetic energy to perform measurements of formation or borehole parameters.
2. Description of the Background
It is desirable for many reasons to transmit electrical signals through the earth as a medium, and to receive the signals at a location spaced from the transmitter. Such a signal system is, for example, used both for the determination of various parameters associated with the medium and for communication purposes. These systems are often used in the investigation of the environment surrounding a borehole, and in particular, the surrounding formations. Various types of borehole logging systems are available to perform these investigations. A class of these systems utilizes electromagnetic field phenomena to obtain data from the environments surrounding the borehole. One type of prior art logging is electrode logging which utilizes an electric field in the surrounding formation to produce a measure of the conductivity of the formation. A conductive mud is necessary for proper use of this system, thus rendering the system inoperative with oil based muds. Inductive logging is another type of prior art electromagnetic logging which uses a time-varying magnetic field in the formation to produce a secondary current flow in the formation. The secondary current flow sets up a second magnetic field which induces current in receiving coils positioned in the borehole, the induced current in the receiving coil or coils being proportional to the secondary current flow in the formation and thus is directly proportional to the conductivity or inversely proportional to the resistivity of the surrounding formation. Using electromagnetic energy for investigating the environment around a borehole is the subject of the present invention.
In the art of well logging, it is well known that it is desirable to be able to measure the parameter of interest, for example, formation resistivity, at different radial distances from the borehole. This is commonly referred to as making measurements at different depths of investigation.
For example, in U.S. Pat. No. 3,453,530 to G. Attali, there is a general discussion of induction logging and of the need for multiple depth investigation measurements, at least as far as they pertain to wireline logs. In providing such a showing, there is a disclosure of producing simultaneous resistivity measurement of three radially different formation zones. Also, the patent recites that when relatively thin formations are encountered, more than one type of formation may enter into the measurement being made at any given moment and that this same problem is encountered at the boundary between two different formations. From this disclosure, those skilled in the art will recognize that it is highly desirable that all measurements be made with as fine a bed resolution as possible and that the bed resolution of all the sensors be closely matched. It is also clear that the Attali system shows the use of the same transmitters for different receiver systems.
In U.S. Pat. No. 3,893,020 to R. A. Meador and L. Thompson, there is a teaching of the use of two transmitters at different frequencies with a single receiver but which works with frequencies considerably higher, perhaps an order of magnitude, than those used in accordance with the present invention. Moreover, in the disclosure of Meador and Thompson, the signals from the two transmitters are received at the receiver coil simultaneously. In their preferred embodiment, Meador and Thompson use different frequencies and different transmitter spacings to concentrate the electromagnetic field at the same depth in the formation in order to calculate the dielectric constant and conductivity of a portion of the formation. Measurements at two frequencies but at the same depth in the formation are necessary since in the disclosure of Meador and Thompson, all measurements are based solely on the received amplitude of the signal.
U.S. Pat. No. 4,319,192 to R. Chemali and J. Tabanou, as well as U.S. Pat. No. 4,107,597 to Meador et al; U.S. Pat. No. 3,551,797 to Gouilloud et al; and U.S. Pat. No. 4,209,747 to Hutchital, are typical of many patents, some with multiple frequencies, that show the use of multiple transmitters with multiple receiver pairs to obtain multiple depths of investigation.
U.S. Pat. No. 4,651,101 to T. D. Barber, R. N. Chandler, and J. F. Hunka relates to the use of a system which, while claiming to be an improvement in electromagnetic wave propagation logging, relates to what the patentees claim to be a "quasi-static electromagnetic field". This patent indicates, in error, that only when the preferred frequencies of between about 10 and 400 KHz and not above 400 KHz are used, displacement currents are insignificant. When displacement currents are significant, the output signal is responsive not only to a conductivity of the formation but also to its dielectric constant, which would be undesirable for the purposes of the present invention. They indicate further a preference that the upper limits of the frequency range be about 200 KHz and that a preferred lower limit of the frequency range would be about 20 KHz.
In U.S. Pat. No. 4,551,789 to Meador, there is a suggestion of the desirability of making a plurality of radially different measurements at a single vertical depth, but no disclosure of how to accomplish this goal.
U.S. Pat. No. 4,818,946 to T. D. Barber uses a particular antenna array and digital processing techniques in an attempt to enhance the resolution of an induction logging tool. Barber states that resolutions of the multi-depth sensors in a conventional logging suite differ, the deep reading sensors having a poorer resolution than the shallow reading sensors. Barber's method requires at least one transmitter and at least two receivers.
U.S. Pat. No. 4,837,517 to T. D. Barber shows the use of one or more transmitters with two or more receivers.
U.S. Pat. No. 4,873,488 to T. D. Barber, R. N. Chandler, and J. F. Hunka is another example of a system using at least one transmitter with at least two receiver arrays.
In U.S. Pat. No. 4,899,112 to B. Clark, J. Jundt, M. Luling, and M. O. Ross, there is described a system for determining formation resistivity at both shallow and deep depths of investigation, but which is dependent upon the long-recognized phenomena that measuring phase shift between a pair of receivers provides a different depth of investigation than measuring amplitude attenuation between that same pair of receivers, quite unlike the present invention in which different depths of investigation can be achieved by using either the amplitude attenuation or the phase shift alone to provide different depths of investigation. Moreover, although this reference shows a plurality of transmitters, they are equally-spaced from the receiver pair, on opposite sides of the receivers.
In the paper entitled "Invasion Profile from the Digital Induction Log" presented by P. A. S. Elkington and H. K. Patel at the SPWLA 26th Annual Logging Symposium on Jun. 17-20, 1985, there is a general discussion of the use of one transmitter and four receivers. The paper discusses the desirability of producing logs from different investigation depths with the same vertical response and resolution and presents evidence of having achieved that goal using a combination of digital processing and mutual inductance cancellation.
In the paper entitled "Introduction to the High Resolution Induction Tool" by R. Strickland, P. Sinclair, J. Harber, and J. DeBrecht, presented at the SPWLA 28th Annual Logging Symposium on Jun. 29-Jul. 2, 1987, there is a disclosure that "These prototypes are of entirely new mechanical construction in which every turn of every coil is positioned precisely along a mandrel made of highly temperature-stable materials. The position of each turn was calculated to precisely zero the mutual inductance with no extra adjustment". This language implies a plurality of receiving antennas, as is discussed in the prior art of the patents listed above where mutual inductance is discussed. There is an apparent inconsistency within the paper because a later statement recites that "All three measurements are made at the same point so that depth shifting errors are eliminated". The statement above with respect to mutual inductance is apparently incompatible with the statement that all three measurements are made at the same point since the use of multiple receivers implies that measurements cannot be made simultaneously at the same depth. It may be that the digital signal processing techniques used with this prior art high resolution induction sensor makes it possible, for all practical purposes, to claim that the measurements are made at the same depth but this is only an inference. By the manner in which the measurements are made, they cannot be made simultaneously at the same point. It should be clear that the paper does not directly state that the measurements are made simultaneously at the same point, but there can be no other reason to address this issue since most tools can make measurements at the same point, but at different times.
Another method uses multiple frequencies, as, e.g. in "Resistivity profiling with a Multi Frequency Induction Sonde", David F. Allen and Scott J. Jacobsen, presented at the SPWLA 28th Annual Logging Symposium, Jun. 29-Jul. 2, 1987. A variation on this theme is the use of a pulsed induction logging tool such as that produced by MPI, Inc., 4174 Technology Drive, Freemont, Calif. 94538. This sensor simultaneously transmits at a wide range of frequencies.
In the paper entitled "Applications of the High Resolution Deep Investigation Resistivity Instrument", presented by R. A. Khokhar, T. D. Lawrence, and W. H. Fertle at the SPWLA 12th French Section (SAID) Int. Formation Evaluation Symposium, Transaction Paper No. K, 1989, there is a discussion dealing with a lateral, pad contact type of device in which it is stated that a bed resolution of 0.5 inch can be obtained with their tool while a resistivity of beds 1.0 inch thick or better can be obtained. This is an example of the difference between resolution and full bed response with the use of two distinctly different tools to make the multiple depth measurements. The paper does clearly identify a need to provide measurements at multiple depths but with a common bed resolution which is as high as is practical.
In the paper entitled "Field Test Results of the High Resolution Induction", presented by M. W. Alberty and D. S. Epps, presented at the SPWLA 29th Annual Logging Symposium on Jun. 5-8, 1988, it is made quite clear that it is highly desirable to decouple the vertical and horizontal bed responses.
In yet another paper entitled "Advances in High Resolution Logging", published in the Technical Review, Volume 36, No. 2, pages 4-14, there is a discussion of the phasor induction tool. In particular, the paper shows that the high resolution of their sensor is obtained, not directly from the deep reading portion of the sensor, but from the shallow reading portion of the sensor. High resolution information thus obtained is used to synthetically improve the resolution of the deep reading portion of the sensor.
In the paper entitled "Vertical Enhancement by Combination and Transformation of Associated Responses", presented by P. A. S. Elkington, J. R. Samworth and M. C. Enstone at the SPWLA 31st Annual Logging Symposium on Jun. 24-27, 1990, there is discussion of there being a fairly sharp distinction between bed resolution and bed response. This distinction is in accord with the comments made above with respect to U.S. Pat. No. 4,818,946. In this paper, a general method of enhancing the vertical response of a wide class of sensors is discussed. The point of the paper is that there are often features visible in a log (resolved), but which are not presented on the log at anywhere near their true value (fully developed). It is thus clear that features are visible in a short spaced sensor which are not in a longer space sensor because, with the types of sensors considered, bed resolution decreases as the depth of investigation increases.
In the paper entitled "Theory of Microinduction Measurements" presented by W. C. Chew and R. L. Kleinberg in the IEEE Transactions on Geoscience and Remote Sensing, Vol. 26, No. 6, November 1988 at pages 707-719, there is the discussion of the use of an induction-type measurement made with a very shallow depth of investigation. This paper discloses that for the extremely small dimensions of the sensor, it is possible to make a crude approximation to the sensor response using geometrical factor theory, but that the response is best understood in terms of what the authors refer to as a "full wave" theory. The frequency of induction of the microinduction sensor is 25 MHz and the plane of the transmitting loop antenna is parallel to the borehole wall. In fact, although considerable space is devoted in the paper to the use of the geometric factor theory versus a "full wave" theory, the paper itself teaches away from the present invention. This is quite clear from a statement on page 708 of the paper, in the three paragraphs prior to Section II.
Finally, in U.S. Pat. No. 4,940,943 to R. P. Bartel and T. F. Rodney, assigned to the assignee of the present invention, there is a teaching of using a single transmitter with a pair of receivers in which the transmitter loop antenna is located in a first cutout within the conductive housing and the receiver antennas are located in additional cutouts in the conductive housing, such cutouts in the conductive housing being used to affect the patterns of the electromagnetic energy from the transmitter to the receivers.
In summary, the majority of the prior art, in attempting to measure formation resistivity at different depths of investigation, uses a plurality of transmitters with a plurality of differently-spaced receivers. In measurement-while-drilling (MWD) applications, in which the sensors are carried in the drill string, a plurality of differently-spaced receivers can cause severe problems due to the effect of "invasion", i.e., the time-dependent movement of borehole fluid into the formation. By way of example, if the hole is being drilled at a rate of penetration (ROP) of 5 ft/hour, and a couple of receiver pairs are spaced 21/2 feet apart, the time elapsed between the measurements taken by a first pair (R.sub.1 /R.sub.2) and the second pair (R.sub.3 /R.sub.4) at the same vertical depth in the borehole is 30 minutes, during which time fluid in the formation can move a considerable distance and thus affect the resistivity measurement. Moreover, those prior references using a plurality of differently spaced transmitters with a single receiver pair, for example, U.S. Pat. No. 3,893,020 discussed above, are used to concentrate the electromagnetic field at the same radial depth of investigation, a considerable departure from the present invention of providing for different depths of investigation.
It is therefore the primary object of the present invention to provide new and improved method and apparatus for logging the resistivity of formations surrounding a borehole, at multiple depths of radial distance from such borehole;
It is also an object of the invention to provide such measurements at essentially fixed vertical depths;
It is also an object of the invention to provide substantially the same bed resolution for each such measurement;
It is also an object of the invention to provide a new and improved method and apparatus which provides multiple depths of investigation with a reduced number of antennas, thus providing a tool configuration which is shorter and more reliable;
It is also an object of the invention to utilize the geometric factor theory in logging formations surrounding an earth borehole using electromagnetic energy; and
It is yet another object of the invention to provide for optimal operation under a variety of borehole conditions.